THE ADSORPTION AND PRECIPITATION OF PHOSPHATE ONTO CALCITE

Scanning electron micrographs show that the reaction products of solution phosphate and calcite are hemispherical, coral-like growths on calcite surfaces. Electron probe micro-analysis indicates that these are a calcium phosphate, and solution data plotted against solubility isotherms suggest that dicalcium phosphate (DCP) is formed rapidly and slowly changes to octa-calcium phosphate (OCP). X-ray diffraction shows that DCP and OCP are present with DCP predominating. The ratio of Ca:P in the treated calcite, after allowing for the CaCO₃ present, is that in DCP, and a surface coating of OCP on DCP is likely. The exchangeability of the reacted P falls from 100 per cent for small amounts (0–10 μg P per g calcite) to a constant 30 per cent when larger amounts are present (200–1000 μg/g) resulting from the porous structure of the coral-like growths. These cover only a small fraction of the calcite surface even when large amounts of P are present so allowing calcite to control solution pH.     

THE HIGH- AND LOW-ENERGY PHOSPHATE ADSORBING SURFACES IN CALCAREOUS SOILS

The two-surface Langmuir equation was used to study P adsorption by 24 calcareous soils (pH 7.2-7.6; 0.8-24.2 per cent CaCO₃) from the Sherborne soil series, which are derived from Jurassic limestone. High-energy P adsorption capacities (x″_m) ranged from 140–345 μg P/g and were most closely correlated with dithionite-soluble Fe. Hydrous oxides therefore appear to provide the principal sites, even in calcareous soils, on which P is strongly adsorbed (x″_m 6–51 ml/μg P). The low-energy adsorption capacities (x″_m) ranged from 400–663 μg P/g and were correlated with organic matter contents and the total surface areas of CaCO₃ but not with per cent CaCO₃, pH, or dithionite-soluble Fe. Total surface areas of CaCO₃ in the soils ranged from 4.0 to 8.5 m²/g soil. Low-energy P adsorption capacities agree reasonably with values (100 pg P/m²) for the sorption of phosphate on Jurassic limestones but phosphate was bonded much less strongly by soil carbonates (k″= 0.08–0.45 ml/μg P) than by limestones (k～10.0 ml/μg P). Low-energy P adsorption in these soils is tentatively ascribed to adsorption on sites already occupied by organic anions (and probably also by bicarbonate and silicate ions) which lessen the bonding energy of co-adsorbed P.     

Alkalinity of virgin solonetzes in northern Kalmykia (the Arshan-Zel’men Experimental Station)

The alkalinity of virgin solonetzes of the Ergeni Upland, Ergeni Plain, and Sarpinsk Lowland has been studied. These soils are characterized by the neutral salinization and the high alkalinity of the solonetzic and subsolonetzic horizons. The analysis of the soil water extracts demonstrated that the highest alkalinity is typical of the subsolonetzic horizons containing calcium carbonates (the B2 and BCca horizons). In the solonetzic horizons without CaCO₃, the alkalinity is lower despite the high exchangeable sodium percentage (up to 42%). The alkalinity of the solonetzic and subsolonetzic horizons may be conditioned by two processes: (a) the hydrolysis of the exchange complex (EC) containing sodium (EC-Na + H₂O ↔ EC-H + Na⁺ + OH⁻) and (b) the reaction of the ion exchange with the substitution of calcium for sodium in the exchange complex (EC-2Na + CaCO₃ ↔ EC-Ca + 2Na⁺ + CO₃²⁻). Calculations performed on the basis of the thermodynamic equations of the physicochemical equilibria according to the LIBRA program indicate that soda is absent in the solonetzic horizons, whose alkalinity is related to the carbonatecalcium equilibria. The high alkalinity of the calcareous subsolonetzic horizons is related to the presence of soda in combination with CaCO₃. The formation of soda in these horizons is due to the reaction of ion exchange described by Gedroits.     

Untersuchungen zur Charakterisierung des pflanzenverfügbaren Phosphats in Thüringer Carbonatböden

Investigations on the characterization of plant available phosphate in Thuringian calcareous soils Phosphate availability in Thuringian calcareous soils and its characterization by CAL‒, NaHCO₃-, H₂O- and CaCl₂-soil tests was investigated in laboratory and pot experiments. Soil CaCO₃ contents > 10% increase the pH value of CAL solution and thus decrease phosphate extractability. The increase of pH also causes an inadequate assessment of plant available phosphate by H₂O- and CaCl₂-soil tests. The CAL soluble P content of the soil corrected by the pH value of the extraction solution was most suitable for the forecast of P uptake of corn in a pot trial. From both parameters P availability indices for calcareous soils can be calculated which are comparable with those in soils containing < 5% CaCO₃. The equation, which still has to be verified in field experiments, reads as CAL-P_corrected = CAL-P_measured · (1 + 0.83·〈pH-value_{CAL solution} — 4.1〉).     

POTASSIUM RESERVES IN A SANDY CLAY SOIL FROM THE SAXMUNDHAM EXPERIMENT: KINETICS AND EQUILIBRIUM THERMODYNAMICS

Potassium-calcium exchange equilibria in, and the kinetics of K release from, soil from the Nil and PK treatments of the Saxmundham Experiment, Rotation 1, and the Ga, Ca + K and K saturated panicle size fractions of the soils were investigated. The free energy and enthalpy ot exchange showed K preference in all the solids. Selectivity for K decreased with increasing particle size and pre-treatment with K salts. A comparison of the ‘differential enthalpy of exchange: per cent K saturation’ relationship for the whole soil with those of the particle size fractions suggests that soil dispersion during particle size separation also decreased K selectivity significantly. When related to the mineralogical composition of the soil, the differential enthalpy data suggest that maximum K. selectivity is associated with a vermiculite/smectite component of inter-stratified minerals in the soil. Isotopic exchange using ^4sCa on the decalcified solids showed a measurable rate of exchange for the Ca forms of the <0.2 and 0.2–2 /μm fractions. This is attributed to traces of blocking materials (CaCO₃ or hydroxy-aluminium polymers), which are removed or rendered porous during treatment with dilute KCl solution, so that isotopic exchange with Ca is then much more rapid. The kinetics of K extraction with a Ca saturated resin, interpreted on the basis of a three-compartment model, suggest that sorbed K and K released by ‘fast’ and slow processes, representing sites of low and high K selectivity, were associated with minera-logically distinct phases in the various particle size fractions.     

Phosphorus sorption and availability indices as affected by properties of calcareous soils

Abstract

The Olsen solution is usually considered the best extractant for estimating P availability in calcareous soils, but predictability of the response to P fertilizers is often low under field conditions. In this study, soil characteristics influencing P sorption and extractability were evaluated. Forty‐one soils varying in CaCO₃, pH, and clay content were selected from pastures to minimize the effect of recent P additions. A P sorption index (PSI) determined from a single addition of 150 mg P/100 g soil was related to soil Ca and CaCO₃, but the correlation coefficients were rather low (r = 0.46 and 0.38, respectively). A P availability index (PAI), determined from the increase in extractable soil P after adding 50 mg P/kg to a suspension and allowing it to dry, was correlated quite well with cation exchange capacity and clay content (r = ‐0.61 for each) in soils with pH < 8.8. The PAI also had a positive relationship with the density of the processed soil sample (r = 0.60). The relationship between PAI and soil Ca (r = ‐0.51) was also better than that between PSI and soil Ca. Inclusion of initial soil P and organic carbon along with CEC increased the predictability of PAI from 37% to 59%. In soils with pH > 8.8, soil pH was the dominant factor controlling the PAI (r = 0.92).     

DESORPTION OF PHOSPHATE FROM SOILS USING ANION EXCHANGE RESINS

After 100h reaction with Cl-resin and 300h reaction with HCO₃-resin (approaching equilibrium), the concentration of anions complementary to phosphate was the critical variable affecting the transfer of P from soil to resin. Solution concentrations of H₃0⁺, Ca²⁺ and phosphate indicated that desorption of P by OH^-, and dissolution of Ca phosphates, controlled P release from soils. P extracted by HCO₃-resin was much greater than by Cl-resin from an acid soil, due to lower total anion and higher desorbing anion concentrations, but there was little difference between the two resins with a calcareous soil. HCO₃-resin extracted a constant proportion of isotopically-exchangeable P from different soils whereas Cl-resin did not. Anion exchange resins provide a convenient means for producing P desorption curves for soils.     

Effect of ground water high in Mg on cation exchange of Na-illite in the presence of CaCO3

Ground waters with electrical conductivity of 4.9–7.4 mmhos/cm, a pH of 7.7–8.8 and a Mg/Ca ratio of 0.8–41.5 were equilibrated with a Na-illite in the presence and absence of CaCO₃. In the presence of CaCO₃, exchangeable Ca, exchangeable $\text{Ca+Mg}$ and the exchangeable Ca/Mg of the equilibrated clay increased but exchangeable Mg and Na decreased. The pH values of the ground waters were positively correlated with (1) exchangeable Ca, (2) exchangeable $\text{Ca+Mg}$, and (3) exchangeable Ca/Mg ratio, both in the presence and absence of CaCO₃.     

在DAP-CaCO3体系与DAP-石灰性土壤体系中NH3的挥发与磷的吸附之间的关系

本工作是研究在CaCO₃体系和石灰性土壤体系中NH₃的挥发与磷的吸附之间相互作用的化学变化,结果表明:在NH₄Cl—CaCO₃体系中,通气的pH值比不通气的低,但溶液中Ca²⁺浓度正好相反。在K₂HPO₄-CaCO₃体系中,在24小时内,通气与不通气的,CaCO₃吸附磷没有差异。在24小时反应期间,在DAP-CaCO₃体系中,因溶液pH值不断增高,NH₃的挥发对CaCO₃吸附磷的影响也就逐渐降低。在石灰性土壤体系中,施用尿素加过磷酸钙或单施尿素时,几乎没有发现NH₃的挥发,而施DAP时,在6天后,NH₃的挥发损失占加入的56%,且DAP处理的土壤,其水溶性磷未通气比通气的高。尿素加过磷酸钙处理的,其水溶性磷通气与未通气的没有差异。另外,尿素加过磷酸钙或过磷酸钙单独处理的土壤,水溶性磷含量均相同。所有这些均表明,在石灰质体系中,NH₃的挥发(如果发生的话)能够加强CaCO₃对磷的吸附,而磷的吸附又能加强NH₃的挥发,两者是相互影响和相互促进的过程。     

Phosphate adsorption by soils 1. Influence of time and ionic environment on phosphate adsorption

Abstract

The influence of reaction time and ionic environments, on phosphate adsorption were studied using one calcareous soil from India, and one calcareous and two latosols from Hawaii.

Phosphorus adsorption by soils has a initial rapid phase followed by a slow process. For plant nutrition studies, where emphasis is on P concentration of solutions from which plants derive P, isotherms should be constructed using data obtained after near‐equilibration has been attained. This condition does not obtain in a few hours and may require 6 days or more.

Calcium chloride as suspending electrolyte always gave lower phosphate solubility than when KC1 was used as electrolyte. Phosphate retention increased with increasing ionic strength. The necessity for obtaining clear supernatant solutions and the desirability for maintaining reasonable constant equilibrium conditions make 0.01 M CaCl₂ a reasonable choice for constructing P sorption isotherms, even though 0.01 M CaCl₂ is not representative of Ca concentrations in many soil solutions. Saturation extracts of soils investigated here were in the range 0.0002 to 0.005 M Ca.

Adsorption of calcium by highly weathered soils was high suggesting specific adsorption. Calcium adsorption was increased by phosphate additions to a Hydrandept.     

Comparing tests for soil fertility: 1. Conversion equations between olsen and mehlich 3 as phosphorus extractants for 120 soils of south italy

Abstract

The multiple‐element extractant Mehlich 3 (M3) has not been tested extensively in Europe. In this Land, soil‐P test recommendations are based, since decades, on the evaluation of the Olsen‐extractable P, and the optimal soil‐P levels have been established to range between 1.5 and 3.0 mg of Olsen‐P per 100 g of soil. A research programme was started in order to assess the suitability of M3 as routine soil‐P test in European laboratories. As a first approach, we develop conversion equations from Olsen‐P to M3‐P, in order to assess the agreement and the consistency of the measurements under a wide range of chemical and physical soil properties. To this aim, 120 samples with drastically contrasting features were selected within 206 soils collected from all the regions of South Italy. Soil‐P ranges were 0.07–60.53 for M3, and 0.08–21.47 mg/100 g for Olsen. The results showed that M3‐P was a P extractant more efficient than Olsen. The amounts of M3‐P were, on the average, twice as large as the Olsen‐P ones, with mean values of 5.70 and 2.75 mg/100g, respectively. The soil properties exerted a great influence, as well as showed a contrasting effect, on the extraction efficiency of each method. For neutral‐alkaline calcareous soils, the average M3‐P/Olsen‐P ratio increased to 2.52, and the efficiency of M3 poorly varied according to soil pH and CaCO₃ content. On the contrary, in CaCO₃‐free acidic soils, the M3‐P/Olsen‐P ratio decreased to 1.63. In particular, anomalous ratio values less than 1.0 were observed for acidic soils with high content of organo‐mineral complexes with be shifted to 3.7–7.7 for calcareous soils, and to 2.7–4.9 for CaCO₃‐free soils. Field calibrations would give more information to establish the proper values according to either the soil properties and plant requirements. The results encourage the introduction of M3 as routine soil‐P test in our Countries. One must take into account, however, that some soil properties, in particular the CaCO₃ content, migth be considered for a more precise comparative evaluation with existing laboratory data.     

石灰化水稻土的形成

石灰化水稻土在我国约有400多万亩。CaCO₃含量10—20%,CaCO₃/MgCO₃之比10—30不等。土壤板结,有石灰结核,有时形成石灰盘。土壤缺磷,少钾,有效态硼、锌、钼、锰也不足。是华南的一种中低产水稻土。石灰化水稻土中的钙主要来自年复一年的施用石灰和引灌含钙高的石灰岩溶洞水;石灰积聚的条件是土壤pH高,土壤CO₂分压低和通透性不良;根据土壤中石灰含量和¹⁴C测定估算,石灰化水稻土的形成至少有360年以上的历史。石灰化水稻土施用石灰不起中和酸性的作用。所以,必须停施石灰,增施有机肥以根本上改良这种土壤,在未改良前应对症施肥以提高产量。     

Combined Effects on Nitrogen Fertilization and Soil of CaCO3 Contents on Corn Performance in Al-Marj Soil,Libya

A two-year field trial was conducted at Al-Marj Research Center, northeast Libya, during the summers of 1996 and 1997 to examine the effect of nitrogen (N) fertilizers on corn (Zea mays L.) growth in a Libyan soil (fine mixed thermic, Typic Haploxerolls) amended with different calcium carbonate (CaCO₃) levels. Two N fertilizer sources (urea and diammonium phosphate, or DAP) were used at three application rates (0, 80, and 160 kg N/ha). The CaCO₃ treatments were 1%, 6%, and 12% based on the soil weight of a 15 cm furrow slice. A basal phosphorus (P) dose of 46 kg P₂O₅/ha as triple superphosphate was applied to all experimental plots before planting. The experimental plots were arranged in a randomized split-split plot design with three replications. The parameters measured included corn grain weight, plant dry-matter content, leaf contents of N, P, potassium (K), calcium (Ca), and magnesium (Mg). The plant dry matter and corn-grain yield were significantly decreased by CaCO₃, but were significantly increased by N fertilizer rates. Average grain yield dropped from 8 to 3 Mg/ha at 1% and 12% soil CaCO₃ content, respectively. Generally, the N source did not have a significant effect on dry matter or on grain yield. The negative effect of CaCO₃ on yield was associated with concomitant significant reduction in leaf N, P, K, and Mg contents, and an increase in Ca content. However, the leaf levels of these nutrients were considered sufficient for corn growth. Therefore, the reduction of leaf-N by CaCO₃ in fertilized soils might have been the major cause of corn dry-matter and grain-yield reductions.     

On the use of phosphorus solubility parameters for the evaluation of phosphate plant availability

Abstract

Several phosphate solubility and availability parameters were obtained for calcareous soil samples fertilized with superphosphate, partially acidulated phosphate rock and a mixture of the two. Plant response to the fertilizer application was studied in a greenhouse experiment. The ionic product of calcium carbonate phosphate [Ca₃(HCO₃)₃PO₄] correlated best with the plant response parameters. The correlations of the plant response parameters with the other solubility parameters (CaPO₄, H₂PO₄ or HPO₄ potentials) were higher than that obtained with conventionally determined available soil phosphorus (P).     

A rapid method to determine cation exchange capacity and exchangeable bases in calcareous,gypsiferous, saline and sodic soils

Summary

A simple, single‐step extraction with LiEDTA for the estimation of CEC and exchangeable bases in soils has been developed. Multivalent cations are stripped from the soil adsorption sites by the strongly chelating agent EDTA, and are replaced by Li. In soils without CaCO₃ or water soluble salts, exchangeable divalent cations (Ca, Mg) are chelated by EDTA and exchangeable monovalent cations (Na, K) are replaced in a single extraction step using 0.25–2.5 g of soil and 10.0 ml of extractant.

In calcareous soils the CEC can be determined in the same way, but for the extraction of exchangeable Ca and Mg, another separate extraction is needed because dissolution of calcite by EDTA is unavoidable. This extraction is done with as much NaEDTA as needed to extract only exchangeable Ca and Mg in a 1:2 (m/V) soil/alkaline‐50% (V/V) aethanolic solution to minimize dissolution of calcite.

In gypsiferous soils gypsum is transformed into insoluble BaSO₄ and soluble CaEDTA by LiBaEDTA thus avoiding interference of Ca from dissolution of gypsum, which renders the traditional methods for determining CEC unsuitable for such soils. To determine exchangeable Ca and Mg, Na₄EDTA is used as for calcareous soils.

In saline/sodic soils replacement of Na by Li is incomplete but the Na/Li‐ratio at the complex after extraction is proportional to the molar Na/Li‐ratio in the extracts, so that the CEC and original exchangeable sodium (ESP) content can be calculated. Additional analysis of Cl and, if necessary, SO₄ in the extracts of saline soils can be used to correct for the effect of dissolution of the salts on the sum of exchangeable cations.

This new method is as convenient as the recently developed AgTU (silverthiourea), but is better suitable for calcareous and gypsiferous soils.     

Suppression of superphosphate-phosphorus fixation by farmyard manure II. Some studies on the mechanisms

Studies were conducted on a calcareous loam soil to find out the effect of premixing superphosphate (SUP) with farmyard manure (FYM) at a 1 to 4 ratlo on the NaHCO₃ extractable phosphorus, lime, and phosphate potentials, and the conversion of SUP to various phosphatic forms. SUP was applied at rates of 98, 245, 490, and 2453 kg P/ba. The soil was incubated for various time intervals from 0 to 96 days.

At all the time intervals, NaHCO₃ extractable phosphorus remained at a higher level in the premixed treatments than that found in the unmixed or SUP treatment only. The soil solution in the premixed treatments contained Ca phosphates of higher solubility than those in the unmixed treatment. The content of Al, Fe-P, and NH₄Cl extractable-P (soluble-P) in the soils of premixed treatments were found to be greater, occurring at the expense of Ca-P. Octocalcium phosphate appeared to be a fairly stable form in all these soils.

The implications of these studies on the availability of phosphorus to plants have been discussed.     

Quantification of individual phosphorus species in sediment: a sequential conversion and extraction method

Common sequential phosphorus (P) extraction methods are not specific to particular chemical species and have several limitations. This work presents the first chemical method for quantification of individual mineral and sorbed P species. It was developed by combining a conversion technique with a sequential extraction procedure. Mangrove sediments with different characteristics were incubated in pH‐adjusted 0.01 M CaCl₂ with and without reference material additions of octacalcium phosphate (Ca₈H₂(PO₄)₆·5H₂O; OCP), hydroxyapatite (Ca₅(PO₄)₃OH), strengite (FePO₄·2H₂O) or variscite (AlPO₄·2H₂O). The changes in soluble phosphate concentration were measured in the supernatant solution, while pH‐induced variations in P composition were determined by subsequent sequential extraction of the sediments. Dissolved phosphate concentration was controlled by adsorption below pH 7.8. Above this pH, soluble phosphate concentration was governed by OCP, which was qualitatively determined by plotting the experimental values of pH + pH₂PO₄ and pH – 0.5 pCa on a solubility diagram including the isotherms of known crystalline phosphate compounds. In contrast to the often‐predicted slow dissolution rate of crystalline phosphates in soils or sediments, drastic changes in P composition by dissolution, precipitation and adsorption processes were detected after 7 days. These were mainly not observed indirectly by changes in dissolved phosphate through adsorption effects, but were determined quantitatively by subsequent sequential extraction, thus enabling the quantification of individual species. Evaluation of the method was performed by standard addition experiments. Besides P species quantification, the method provides the means for other applications, such as the determination of P mineral dissolution kinetics in soils and sediments, the prediction of P composition in changing environmental settings and the refinement of theoretical models of phosphate solubility in soil and sedimentary environments.     

Effect of iron oxide on phosphate sorption by calcite and calcareous soils

Pure calcite (AR grade CaCO₃) was treated with ferrous perchlorate solution to give a surface coating of iron (Fe) oxide. Maximum sorption (x_m) of phosphate (P) by the calcite increased from 18.2 to 160 mg P kg^?1 as the amount of coating increased from 0.00 to 16.0 g Fe₂O₃, kg^?1 CaCO₃. Evidence for Fe oxide coatings on carbonate minerals in two Sudanese soils was obtained by optical microscopy and electron-probe microanalysis. The relative contributions of carbonate and Fe oxide minerals, and Fe oxide coatings to P sorption in these soils were calculated, based on an assumed model of oxide distribution. Separate-phase Fe oxide was the major contributor (30–40%) to P sorption in the soils; the Fe oxide coatings on carbonate minerals were only minor contributors (< 6%), and the contribution of uncoated carbonate minerals was found to be negligible (<1 %). These results suggest a very minor role for carbonate minerals, even when coated with Fe oxide, in the sorption of P by these calcareous, Sudanese soils.     

Effect of Cation Species on the Desorption of Phosphorus in Soils Treated with Carbonate

Three-month incubation of soils with K₂CO₃ raised the pH about the same as did an equivalent amount of CaCO₃ but was substantially more effective in enhancing the water solubility of P (4–14-fold at the solution to soil ratio of 60:1). In the K₂CO₃-treated samples, the KCl added to achieve a 25% increase in ionic strength caused an 11–44% drop in the water-soluble P. However, if the water to soil ratio increased, the P desorption in the CaCO₃ amended samples approached that in the K₂CO₃-treated soils, and the KCl-induced reduction in water-soluble P diminished. These responses to changes in the extraction ratio were taken to indicate that the cation species of the liming material and the ionic strength in the matrix solution affected the rate more so than the absolute quantity of P desorption. The significance of these factors under field conditions was discussed.     

Phosphate sorption by jurassic oolitic limestones

Sorption isotherms and calcium phosphate solubility data were used to interpret the processes responsible for sorption of phosphate on three Jurassic oolitic limestones whose surface areas ranged from 1.0 to 1.5 m²/g CaCO₃. It was concluded that total sorption was due to (1) an initial chemisorption reaction at final P concentrations below 0.5 μg/ml, with Langmuir adsorption parameters of about k = 10 ml/μg P and x_m = 100 μg P/g; above this concentration most of the additional sorption was due to (2) phisical adsorption (of very low bonding energy) which may occur in multilayers on the initial chemisorbed layer, and (3) precipitation, probably of octacalcium phosphate, Ca₈H₂(PO₄)₆.5H₂O, from solution and its deposition on the adsorbed layers. The last two reactions probably overlap and cannot be distinguished from each other.